Ionospheric and Birkeland current distributions inferred from the MAGSAT magnetometer data

Abstract

The MAGSAT vector magnetometer data, with the aid of an accurate magnetic field model, have been used to infer ionospheric and field‐aligned sheet current density distributions. The MAGSAT spacecraft is uniquely suited for this study because it provides, for the first time from a satellite, simultaneous observations of effects due to both ionospheric and Birkeland current systems. The method emphasized here consists of a Fourier transform technique developed for use on surface magnetometer data and recently used by Baumjohann et al. (1979). Data acquired by MAGSAT on two days in 1979 were analyzed in detail. The first was December 21, 1979, a reasonably quiet (Σ Kp = 8−) time, when data acquired in the sunlit hemisphere (with presumably uniform conductivity) were analyzed. The second example deals with a rather active time, November 13, 1979, (Σ Kp = 35+) in the northern hemisphere, with more complicated conductivity and current distributions. By comparing Hall current densities inferred from the MAGSAT data and those inferred from simultaneously recorded ground‐based data acquired by the Scandinavian magnetometer array, it is found during two other events that the former are underestimated due to the considerably high damping of magnetic variations with high spatial wave numbers between the ionosphere and the MAGSAT orbit. Important results of this study include the fact that the Birkeland and electrojet current systems are colocated. In addition, more realistic ionospheric current distributions have been developed. The analyses have shown a tendency for triangular rather than constant electrojet current distributions as a function of latitude, which is also consistent with the statistical, uniform region 1 and region 2 Birkeland current patterns.